The presence of Bifidobacterium in an infant's intestine is typically an indication of a healthy intestinal bacterial flora. While types and sizes of Bifidobacterium populations vary with age, bifidobacteria are present throughout the life cycle of a human being.
Bifidobacteria have various physiological properties. For instance, these bacteria can inhibit putrefactive and pathogenic bacteria, maintain a normal intestinal bacterial population, inhibit production of toxic amines, synthesize vitamin B groups, and produce L-lactic acid.
Since bifidobacteria tend to be more anaerobic than other lactic acid-producing bacteria, the development of food products containing viable bifidobacteria has encountered problems such as difficulties in maintaining bacterial viability during storage of the food products. In addition, orally administered bacteria are under the stress caused by, e.g., gastric acid and bile salt encountered in the gastrointestinal environment.
Nearly a hundred different kinds of bifidobacteria-containing food products have been developed to date, yet none of the strains used in these products are tolerant to oxygen, gastric acid, and bile salt. Many strains disclosed in the prior art, for example, are acid- and oxygen-tolerant, yet few of them are bile salt-tolerant (see, e.g., Tables I and II).
According to some studies, Bifidobacterium spp. ATCC 15700, ATCC 15696, ATCC 15697, and ATCC 15707 have extremely low growing abilities when cultured in the presence of 0.3% glycocholate for 24 hours. Studies of acid-tolerant strains have been devoted largely to the bacterial viability in acidic dairy products (pH 4 to 4.8) over a period of storage under refrigerating conditions. Some studies carried out to test tolerance to gastric acid set pH values at about 3 to 3.5 and not at about 2, which occurs during secretion of gastric acid.